Solid state radiometric pulse initiator for remote meter reading systems

ABSTRACT

A solid state radiometric pulse initiator for a remote meter reading system includes a sensing head having two combined source-pickup pairs mounted in a meter for initiating pulses in response to movement of a pattern disc. The source-pickup pairs control an output pulse generator to produce pulses over a three wire telemetering circuit. Each source-pickup pair includes a solid state radiation emitter and a cooperatively arranged radiation responsive solid state switch. The source-pickup pairs are connected to a source of alternating current so as to be alternately energized in an out-of-phase relationship to prevent false triggering due to cross-radiation between the pairs. A radiation filter is provided on a sensing head carrying the source-pickup pairs so as to concurrently increase the triggering sensitivity of the source-pickup pairs and permit operation with reduced current levels when the pulse initiator is operated under strong ambient light conditions typical in certain meter applications.

tates atent 1 1111 3,733,493 McClelland, III 1 ay 15, 1973 541 SOLIDSTATE RADIOMETRIC PULSE 3,251,952 5/1966 Shomer ..250 219 on x INITIATORFOR REMOTE METER 3,499,158 3/1970 Lavine et al ..250/217 SS READINGSYSTEMS Primary Examiner-Walter Stolwein [75] Inventor: TheodoreMcclenand Cary Att0rneyA. T. Stratton, Donald R. Lackey and R.

Warren Smith [73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa. [57] ABSTRACT [22] Filed: Nov. 12, 1971 A solid stateradiometric pulse initiator for a remote pp No: 198,319 meter readingsystem includes a sensing head having two combined source-pickup pairsmounted in a meter for initiating pulses in response to movement of apat- [5 C 9 2 /2 tern disc. The source-pickup pairs control an output 3/9 pulse generator to produce pulses over a three wire [51] Int. Cl...G01d 5/36 telemetering circuit. Each source-pickup pair includes Fieldof Search 201 231 a solid state radiation emitter and a cooperativelyar- 219 ranged radiation responsive solid state switch The 356/114source-pickup pairs are connected to a source of alternating current soas to be alternately energized in an References Cue-d out-of-phaserelationship to prevent false triggering UNITED STATES PATENTS due tocross-radlation between the pans. A rad1at1on filter 1s prov1ded on asensmg head carrymg the 3,290,593 12/1966 Crowdes ..324/l57 Xsource-pickup pairs so as to concurrently increase the 3,399,347 8/1968Martens ..324/96 triggering sensitivity of the source-pickup pairs and3,466,451 1969 Jr CR permit operation with reduced current levels whenthe 3,417,249 12/1968 Akmenkalns et al.... ....250/2l7 SS pulseinitiator is operated under Strong ambient light 3,283,157 11/1966Blackmer .250/239 x conditions typical in certain meter applications 7Claims, 6 Drawing Figures TIZ l6 K [1111111 I 25A izsc 26 25 PATENTEDHAY1 5W5 SHEET 3 BF 3 FIG. 5 \\\m SOLID STATE RADIOMETRIC PULSE INITIATORFOR REMOTE METER READING SYSTEMS FIELD OF THE INVENTION This inventionrelates to pulse initiators for pulse operated remote meter readingsystems and more particularly to a solid state radiometric pulseinitiator including two source-pickup pairs having improved selectivityand sensitivity for detecting markings on a meter driven pattern disc.

BACKGROUND OF THE INVENTION Electronic pulse initiators have beenemployed in remote meter reading systems and are well known fortransmitting meter information of integrating type meters such aswatthour meters into a representative quantity fonned by a repetition ofpulses. Remote meter reading telemetry systems of the type to which thisinvention pertains are described in Electrical Metermans Handbook,Seventh Edition, published in 1965 by the Edison Electrical Institute,New York, N.Y. Photoelectric pulse initiators are described in theaforementioned publication and one type of a pulse initiatormanufactured by the assignee of this invention includes a sensing headmounted within a watthour meter for carrying a pair of lamps whichradiate indicia of a pattern disc movable by a shaft connected to themetering movement. The indicia couples the light of the lamps tophotosensitive semiconductor switches also carried in the sensing head.Two sets of the indicia are oriented on the disc so that thephotosensitive switches are rendered conductive alternately to actuatean output pulse generator for alternately signalling over two circuitsof a three wire telemetering circuit. Since the pulse initiators aremounted in watthour meters intended for outdoor installations, they aresubjected to sunlighted high ambient light conditions. Operation underthese conditions has, in some instances, required adjustment of thesensitivity of the photosensitive semiconductor switches so that theyare susceptible to false triggering due to the high levels of ambientlight or lack of optical shielding. Elaborate shielding or use oflightproof housings greatly increases the cost of such pulse initiatorsand also makes use difficult due to the compact arrangements requiredfor mounting within a meter housing.

SUMMARY OF THE INVENTION In accordance with the present invention, asolid state radiometric pulse initiator for a remote meter readingsystem includes a sensing head having first and second source-pickuppairs with each pair being mounted so as to be periodically alignedrespectively, with a separate indicia marking of a pattern disc rotatedby a meter movement. Radiation emitted by a solid state source of eachpair is separately coupled to a radiation responsive solid state switch,of the associated pair, by an associated marking. The radiationresponsive switches are connected between an output generator and apower source of alternating current in an opposite parallel relationshipfor selectively conducting current in opposite directions to the outputgenerator. The emitter sources are also connected in an oppositeparallel relationship to the power source so as to conduct on oppositephases of the alternating current when the associated switch of eachpair is rendered operative to conduct. During each complete cycle of thealternating current source, current in either direction is capable ofbeing applied to the output generator dependent upon the source-pickuppair being enabled by positioning of the associated marking when theother source-pickup pair is disabled.

Another feature of this invention is a filter provided over the openingof the sensing head to reduce the effective ambient light whileincreasing the sensitivity of source-pickup pairs so as to require lowercurrent levels and less power drain of such pulse initiators.

Other advantages and features of this invention will be apparent uponreference to the detailed description of the preferred embodimentillustrated in the drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a remote meterreading system including a solid state radiometric pulse initiator madein accordance with this invention;

FIG. 2 is a graph illustrating filtered and unfiltered ambient andemitter source radiations relative to triggering levels of switchpickups;

FIG. 3 is a top plan view of the pulse initiator and the associatedpattern disc shown in FIG. 1;

FIG. 4 is an enlarged side plan view illustrating the pulse initiator,the pattern disc and the associated gearing arrangement shown in FIG. 1;

FIG. 5 is a schematic cross-sectional view in perspective of the pulseinitiator sensing head having a filtering lens made in accordance withthe present invention and a portion of the pattern disc in one operativeposition; and

FIG. 6 is an electrical schematic diagram showing a pulse initiatorcircuit made in accordance with this invention and connected between anoutput pulse generator and an alternating current power source.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawingswherein like numerals are used to designate similar or like elements inthe several figures, and more particularly to FIG. 1, a meter 8 havingan electroresponsive meter movement including metering coils such as apotential coil 10 and a current coil 12 and being of the type utilizedin watthour meters, in one preferred embodiment for driving a shaft 14and an associated disc 16. The meter 8 being of the integrating typeregisters the electrical energy quantity of a circuit 17 being measuredby indicating the rotation of the shaft 14 at a remote location forcounting or totalization by means of a remote meter reading system 18. Asolid state radiometric pulse initiator 20 made in accordance with thisinvention, an output generator 22 and alternating current power supply23 are mounted within the housing 24 of the meter 8 for translating theshaft rotation into pulse signals. The output generator 22 is connectedto a three wire telemetering circuit 25 connecting the meter 8, with athree wire remote receiver 26 adapted for use in three wire telemeteringcircuits which are well known and are described in the aforementionedElectrical Metermans Handbook publication and may include, for example,a stepper motor having two input signal windings 27 and 28.

The pulse initiator 20, as shown in FIGS. 3 and 5 includes a sensinghead 30 supporting first and second source-pickup pairs 33 and 34,mounted adjacent a pattern disc 36 driven by a suitable gearing system38 from the meter shaft 14. The pulse initiator 20 further includes apulse initiating circuit 40 described more fully in connection with thedescription of FIG. 6.

The first source-pickup pair 33 is mounted adjacent the secondsource-pickup pair 34 in a cavity 30A of a dual-in-line sensing headpackage as shown in FIG. 5. The cavity opening of the sensing head 30 isdisposed substantially parallel to the pattern disc 36 and is spacedslightly from the top surface thereof. A radiation filter 41 describedhereinbelow, covers the opening of the cavity 30A. First and secondindicia markings 42A and 42B are defined by reflective areas on the disc36 which are oriented radially at different radii so as to be rotatablein different circular paths. Disc rotation orients the reflective areas42A and 428 to overlap the first and second source-pickup pairs 33 and34, respectively, at different angular positions of the disc 36. It isunderstood that the number of reflective areas can be changed to developa desired number of signals for each complete rotation of the disc 36.

Each of the source-pickup pairs 33 and 34 includes a solid stateradiation emitter source 338 and 34S, respectively, and a solid stateradiation responsive switch pickup 33P and 34P, respectively, which areadapted to be triggered to a conductive state in response to apredetermined radiation intensity when properly biased. In a preferredembodiment, each solid state switch pickup includes a phototransistorhaving a radiation responsive base-emitter circuit which when radiatedat a predetermined triggering level, will render the collector-emittercircuit conductive. It is apparent to those skilled in the art that aphototransistor may be replaced by an equivalent device such as aphotosemiconductor controlled rectifier (SCR). The radiation emitter ofeach source-pickup pair 33 and 34 includes a light emitting diode (LED)which characteristically emits radiation in the infrared frequencyspectrum when conducting in the diode forward current direction.

In actual practice, mounting of the source-pickup pairs 33 and 34 withinthe sensing head 30 is such that they are in close side-by-siderelationship, for example, in the order of one-fourth inch so as toprovide a compact mounting arrangement. The pairs 33 and 34 arepreferably shielded from each other within the sensing head package bypartition 43, shown in FIGS. 3 and 5.

Referring now to FIG. 6, wherein the pulse initiating circuit 40 isillustrated schematically and is preferably mounted in the sensing head30. The circuit 40 is supplied alternating current in the order of 7 to8 volts by the power supply 23 including a step-down transformer 45. Theprimary 45P of the transformer 45 is normally connected to a 120 voltconventional power line source provided across a pair of conductors 54.The secondary 458 of the transformer 45 supplies energizing power to thepair of conductors 46 and 47 of the circuit 40.

The output generator 22, being of the magnetic latching type, includes apolarized coil 48 connected in series with the conductor 46 and formsthe output of the circuit 40. The signal output of the output generator22 is provided by a three contact device 49 defining a single pole,double throw switch connected to the telemetering circuit 25 which formstwo circuits including conductors 25A, 25B and 25C. A switch arm 49A ofthe device 49 is magnetically actuated by the coil 48 such that when theconductor 46 is positive relative to the conductor 47, during ahalf-cycle of the output of the output of the power source 23, currentflows through the coil 48 so as to latch the arm 49A in a first or lowerposition for conductively connecting the conductors 25A and 25B. The arm49A is latched mechanically in this position after current flowterminates or with repeated current flow in the same direction asindicated by the current directional arrow 50. When the conductor 46 ispoled negatively relative to the conductor 47, during an opposite halfcycle of the power supply output, current flow through the coil 48 isreversed as indicated by the current directional arrow 51, so as tounlatch the arm 49A and position it such that the conductor 25C isconnected to the conductor 25B. This operation is well known to thoseskilled in the art of output generators for pulse transmission over athree wire telemetering circuit. It is to be understood that the outputgenerator 22 may include a bistable electronic circuit for developingpulse signals in the circuits 25A-25B and 25C-25B.

The solid state switch pickups 33P and 34P of the source-pickup pairs 33and 34 comprise NPN phototransistors connected together in oppositecurrent conducting relationship across the conductors 46 and 47. Theseswitch pickups connect the coil 48 across the power source 23 whenrendered conductive. Also, the radiation emitter sources 33S and 345 areconnected together in opposing parallel relationship and across theconductors 46 and 47 through a resistor 52 as further illustrated inFIG. 6. The resistor 52 is of the current limiting type whichestablishes the level of current applied to each of the radiationemitter sources 338 and 348. In one preferred embodiment, the solidstate switch pickups 33P and 34? are formed by silicon NPNphototransistors responsive to radiation emitted by the emitter sources338 and 34S and over a wavelength range of 0.4 to 1.l microns. Resistors53 and 54, respectively, are connected in the transistor emittercircuits which in turn are connected to the conductors 47 and 46,respectively. The transistor base circuits are connected by resistors 55and 56 to the conductors 46 and 47, respectively, and the transistorcollector circuits are connected in series with diodes 57 and 58 withthe indicated polarity to the conductors 46 and 47 respectively.

When the conductor 46 is established negative relative to the conductor47 by the power supply 23, the switch pickup 34P is forward biased fortriggering from a nonconductive state to a conductive state whenradiated with a predetermined intensity of radiation from the emittersource 348. Current passes through the collector-emitter circuit to thecoil 48 in a direction indicated by the arrow 51. Similarly, when theswitch pickup 33? is radiated with a predetermined level of radiationfrom the emitter source 338 and the conductor 46 is made positive withrespect to the conductor 47 to forward bias the transistor circuits ofthe switch pickup 33P, current flows through the collect-emitter circuitin the direction indicated by the arrow 50. Current flow is applied inan opposite direction to the coil 48 to actuate the three contact device49 as described hereinabove.

The emitter sources 338 and 348 are connected so that when energizedwith a predetermined current level in the forward diode conductingdirection, sufficient radiation thereof will be reflected when theassociated reflective areas 42A or 428 is positioned directly beneaththe portion of the sensing head 30 including the associatedsource-pickup pairs 33 or 34, respectively.

In one embodiment the LED of each source 338 and 348 includes aninfrared emitting diode made of P-type and N-type gallium arsenidematerial having a narrow band of radiation peaking at 9,000A(non-visible) when forward biased. Accordingly, when the half cycle ofthe power supply output is such that the conductor 46 is poled positivewith respect to the conductor 47, the emitter source 338 conductscurrent and the switch pickup 33I is poled for forward biasing whileboth the switch pickup 34F and emitter source 348 are reverse biased anddisabled for current flow. correspondingly, if the conductor 46 is madenegative with respect to the conductor 47 during the opposite half cyclethe source pickup pair 34 is enabled while the source-pickup pair 33 isdisabled and it is impossible to concurrently trigger the solid stateswitch pickup 33?.

Accordingly, the infrared emitter sources 335 and 34S conductalternately with each phase reversal of the alternating current outputof the power supply 23. It is an important characteristic of the lightemitting diodes utilized for emitter sources 335 and 348 that theradiation therefrom is emitted in a narrow radiating cone so that theradiation incident upon the reflective areas 42A and 423 has a spotlighteffect preventing dispersion of the radiation. The radiation isreflected onto the photosensitive transistors forming the solid stateswitch pickups 33P and 34? for triggering since they have a primaryresponse typically between 3,000A to l 1,000A peaking at about 8,000A.Due to the inherent false pulse triggering prevention circuitarrangement as well as the optical shielding of the source-pickup pairs33 and 34, cross-radiation between emitter sources of switch pickups ofdifferent source-pickup pairs is prevented to prevent false triggering.

As noted hereinabove and shown in FIG. 5, the opening to the cavity 30Aof the sensing head 30 package includes a filter 41. The use of thefilter 41 uniquely increases the triggering sensitivity of thesource-pickup pairs 33 and 34 when the pulse initiator is utilized inoutdoor mounting arrangements subject to high ambient sunlightedconditions. The filter 41 is a coating of material formed of a vaporizedsilicon compound having a predetermined filtering characteristic. Theradiation filtering characteristic of this compound has been found tosubstantially block radiation having a wavelength less thanapproximately 8,500A and to pass substantially all radiation wavelengthsabove approximately 8,50OA. The spectral output of the diode emittersources 338 and 348 is within a relatively narrow bandwidth centeredabout 9,000A as noted above and the radiation response of thephototransistor switch pickups 33F and 34? is also within a bandwidthfrom 8,500A and slightly above so that most of the infrared sourceradiation passes out through the filter 41, strikes the areas 42A and42B and passes back through the filter 41 as shown by arrows 59 in FIG.5. It is known that the radiation intensity included in sunlightradiations, indicated by source 60, is not linear and the intensitythereof decreases through higher frequencies having a peak value below8,50OA. It is believed this characteristic is utilized in this inventionsince only a small amount of the radiation of the emitter sources 338and 348 is filtered by the filter 41 while a substantially larger amountof the normal ambient radiation provided by sunlight is filtered withinradiation response spectrum of the switch pickups 33F and 34?.

Reference is made to the graphs of FIG. 2 to describe my manner ofincreasing sensitivity of the sourcepickup pairs in relation to thedifference in radiation intensities of the emitter sources and thenormal ambient radiation of the source 60 which is principally providedby sunlight. In one initial exemplary condition without the filter 41,indicated by Roman numeral I, the triggering level of the solid stateswitch pickups 33F and 34? was provided by adjusting the values of thetransistor biasing resistors such that when an optimum current wasprovided through the emitter sources in the order of fifty milliamps, asdetermined by the value of the resistor 54, a triggering level indicatedby line 62 was established. The diode current in either of the sources338 or 348 produced a radiation intensity indicated by the bar 63extending to the triggering level 62 in the absence of any ambientlight. In this condition the source-pickup pairs 33 and 34 were operatedin a normally lighted ambient including the sunlight radiations of thesource 60 which developed the ambient radiation intensity represented bythe bar 64. This intensity was greater than the triggering level 62rendering the sources 338 and 34S ineffective to selectively operate thepulse initiator circuit. Effective operation was possible by increasingthe diode current of the emitter sources 338 and 34S and decreasing thesensitivity of the switch pickups, however, this was beyond practicallimits for extended reliable service life.

In a second exemplary condition, indicated by Roman numeral II, thefilter 41 was provided over the cavity 30A of the sensing head packageso as to reduce the normal ambient radiation intensity by an amountindicated by the shaded portion 65A of the bar. Due to the concurrentfiltering of the emitter sources radiations, a maximum current level of65 milliamps was required to produce the radiation intensity to triggerthe switch pickups at the sensitivity triggering level 62, which isindicated by the bar 66, being the same as in the original examplecondition. The shaded portion 66A of the bar 66 indicates emitter sourceradiation blocked from the switch pickups by the filter 41 and theremaining solid portion of the bar 66 indicates source radiationtransmitted to the pickups.

It was later observed that extraneous ambient light was filteredsubstantially more than the emitter source radiations and it was foundthat the sensitivity of the source-pickup pairs could be reduced in athird exemplary condition indicated by Roman number III. The sensitivityof the response of the source-pickup pairs 33 and 34 being directlyrelated to the difference in the background or ambient radiation and theradiation of the emitter sources 338 and 348, the triggering level ofthe switch pickups 33P and 34F was decreased to the level indicated byline 67, indicated by being lower in the graph of FIG. 2 than thetriggering level line 62. This increase of sensitivity also permittedthe current of the emitter sources 338 and 348 to be reduced, forexample, to 40 milliamps maximum current to provide radiationintensities indicated by bar 68 extending to the line 67 to trigger theswitch pickups 33F and 341. The shaded bar portion 68A above bar 66indicates the portion of the emitter source intensity blocked by thefilter 41. Accordingly, the use of the filter 41 as describedhereinbefore increases the sensitivity and response of the pulseinitiator circuit and concurrently permits operation of the emittersources 338 and 348 at lower current levels reducing the amount of powerrequired for the pulse initiating circuit and increasing the servicelife of the emitter sources. Also with the increased sensitivity of theswitch pickups 33F and 34?, greater accuracy is provided in response tothe positioning of the reflective areas 42A and 42B adjacently below thesensing head 30.

In addition to the reduction of cross-triggering between thesource-pickup pairs 33 and 34 as described hereinabove, there is alsoreduced tendency of any false triggering due to the normal ambientradiations or extraneous light sources since the filtering lens 41substantially reduces radiation having frequencies effective to causetriggering of the switch pickups 33S and 34S and makes themsubstantially exclusively responsive to the emitter source radiations.Accordingly, the pulse initiator 20 is referred to as a radiometric typeas distinguished from the more general light responsive or photometricpulse initiators used herein before my invention in remote meter readingsystems.

While a preferred embodiment of the invention has been described herein,it is contemplated that modifications and alterations of my inventionmay be made without departing from the spirit of this invention.

I claim:

1. In a system for transmitting information from a meter having a metermovement including a rotating shaft, the combination comprising:

a pattern disc drivenly connected to said shaft, said pattern discincluding first and second indicia markings rotatable through separatecircular paths;

a pulse initiator including first and second radiation source-pickuppairs, each of said first and second pairs including a solid stateradiation emitter source and radiation responsive pickup located so thateach pair is periodically aligned with a separate one of said first andsecond markings for directing radiations of a source to an associatedpickup when said pattern disc is rotated;

an alternating current power source;

first circuit means connecting said radiation emitter sources of eachpair in separate circuit paths across said power source such thatcurrent flows alternately in opposite directions through each path foralternately energizing said sources; and

second circuit means connecting said pickups of each pair in separatecircuit paths across said power source such that current is appliedalternately in opposite directions to each of the last-named circuitpaths when current is applied to an associated states and is connectedwith said power supply so as to i be poled in a forward conductingdirection concurrently with energization of the associated solid stateemitter source.

4. The system of claim 3 wherein a three wire output generator isconnected between said second circuit means and said power source forgenerating two different signals in response to the conducting state ofeach switching device, respectively.

5. The system of claim 3 wherein a common sensing head package exposedto a high level of ambient light radiations supports said solid stateemitter source and said solid state switching device of eachsource-pickup pair, said package including a cavity receiving each pairwith the cavity opening facing said disc; and a filter means coveringthe cavity opening, said filter means having a predetermined frequencyrange for substantially limiting all effective radiations between asource and a pickup except for the source emitted radiations and theresponsive radiations of the pickup.

6. The system as claimed in claim 5 wherein the ambient radiationcondition predominantly includes radiations of the sun and saidpredetermined frequency range is principally within the infraredfrequency spectrum; and further wherein said indicia markings aredefined by radiation refiective areas of said disc.

7. The system as claimed in claim 6 wherein each source-pickup pairincludes an infrared light emitting diode and asilicon phototransistorhaving a radiation frequency response over a slightly greater frequencyrange than the frequency range of radiations of each light emittingdiode and wherein a radiation limiting characteristic of said filtermeans includes a frequency range partially within said radiationfrequency response of each phototransistor and a radiation transmittingcharacteristic including substantially all of the radiation frequencyrange of said diode.

1. In a system for transmitting information from a meter having a metermovement including a rotating shaft, the combination comprising: apattern disc drivenly connected to said shaft, said pattern discincluding first and second indicia markings rotatable through separatecircular paths; a pulse initiator including first and second radiationsourcepickup pairs, each of said first and second pairs including asolid state radiation emitter source and radiation responsive pickuplocated so that each pair is periodically aligned with a separate one ofsaid first and second markings for directing radiations of a source toan associated pickup when said pattern disc is rotated; an alternatingcurrent power source; first circuit means connecting said radiationemitter sources of each pair in separate circuit paths across said powersource such that current flows alternately in opposite directionsthrough each path for alternately energizing said sources; and secondcircuit means connecting said pickups of each pair in separate circuitpaths across said power source such that current is applied alternatelyin opposite directions to each of the last-named circuit paths whencurrent is applied to an associated emitter source in the same pair forselectively enabling one source-pickup pair when the other is disabled.2. The system of claim 1 wherein each of said radiation source-pickuppairs includes a unidirectionally conducting solid state emitter sourceconnected in opposing parallel relationship to the other source.
 3. Thesystem of claim 2 wherein each radiation responsive pickup includes asolid state switching device operative between non-conducting andconducting states and is connected with said power supply so as to bepoled in a forward conducting direction concurrently with energizationof the associated solid state emitter source.
 4. The system of claim 3wherein a three wire output generator is connected between said secondcircuit means and said power source for generating two different signalsin response to the conducting state of each switching device,respectively.
 5. The system of claim 3 wherein a common sensing headpackage exposed to a high level of ambient light radiations supportssaid solid state emitter source and said solid state switching device ofeach source-pickup pair, said package including a cavity receiving eachpair with the cavity opening facing said disc; and a filter meanscovering the cavity opening, said filter means having a predeterminedfrequency range for substantially limiting all effective radiationsbetween a source and a pickup except for the source emitted radiationsand the responsive radiations of the pickup.
 6. The system as claimed inclaim 5 wherein the ambient radiation condition predominantly includesradiations of the sun and said predetermined frequency range isprincipally within the infrared frequency spectrum; and further whereinsaid indicia markings are defined by radiation reflective areas of saiddisc.
 7. The system as claimed in claim 6 wherein each source-pickuppair includes an infrared light emitting diode and a siliconphototransistor having a radiation frequency response over a slightlygreater frequency range than the frequency range of radiations of eachlight emitting diode and wherein a radiation limiting characteristic ofsaid filter means includes a frequency range partially within saidradiation frequency response of each phototransistor and a radiationtransmitting characteristic including substantially all of the radiationfrequency range of said diode.